The invention relates to a gamma correction circuit for a video signal, comprising a linear and a non-linear amplifier circuit and at least a black level clamping circuit, amplifier outputs being connected to inputs of a signal combining circuit comprising an output for carrying the gamma-corrected signal.
Such a gamma correction circuit is described in the Netherlands application No. 6 807 530. It is described that in the signal combining circuit a differential signal is first formed from the linear and the non-linear amplifier signals, which differential signal is thereafter added, to a certain extent, to the linear signal to obtain the gamma corrected output signal.
When the differential signal is being formed it is essential that the same signal level be present in the linearly as in the non-linearly amplified signal, in the presence of the so-called black level or peak-white level. Namely, the differential signal must have the value zero at the black and the peak-white levels respectively. This requirement is very stringent as otherwise, not only the degree of gamma correction is changed but also the peak-peak value of the video signal is affected during a gamma adjustment. In practice this is impermissible; a change in the gamma correction must not affect the peak-peak value of the video signal and, consequently, the contrast range on display.